For most embedded projects, we recommend that you use our highly flexible Xively Embedded C library. It integrates Xively features like Secure File Transfer over MQTT, it's highly portable, and it's been ported to bare-metal, RTOS and OS architectures.
Check it out:
Explore the Official Xively Embedded C client
For server-side MQTT communication (or if you'd just prefer to use a language you are familiar with), any MQTT library will work against the Xively broker. Here are a few clients we recommend in some popular languages:
Create web app back-ends with Django, tackle research and scientific applications with tool kits like NumPy and Matplotlib, and connect Linux systems to Xively from supercomputers down to the Raspberry Pi and BeagleBone.
A ruby bunding against libMosquitto, which we recommend as the easiest way to get set up in a Ruby environment.
A Clojure Codec for MQTT. Provides a netty encoder and decoder for the MQTT protocol.